Record time when last automatic backup was created

Closes #3474

(cherry picked from commit 1aa5222c99f50543c47d5f3bd4bb892e16e31efb)

domain/src/main/java/tachiyomi/domain/backup/service/BackupPreferences.kt

@@ -13,4 +13,7 @@ class BackupPreferences(
     fun numberOfBackups() = preferenceStore.getInt("backup_slots", 2)
 
     fun backupInterval() = preferenceStore.getInt("backup_interval", 12)
+
+    // TODO: move this and other "app state" preferences elsewhere and exclude from backups
+    fun lastAutoBackupTimestamp() = preferenceStore.getLong("__APP_STATE_last_auto_backup_timestamp", 0L)
 }